Sputtering apparatus with sealed cathode-shield chamber

ABSTRACT

FILMS OF MATERIAL ARE DEPOSITED ON A SUBSTRATE AT HIGH RATES BY SPUTTERING IN WHICH A TARGET IS BOMBARDED BY IONS IN A LOW PRESSURE GLOW DISCHARGE IN THE PRESENCE OF A MAGNETIC FIELD. A VERY LOW PRESSURE IS PROVIDED BY A VACUUM PUMP IN A SEALED CHAMBER BETWEEN THE CATHODE   AND THE GROUND SHIELD TO PREVENT ARCING AT HIGH POWER DENSITIES. ATMOSPHERIC PRESSURE MAY ALSO BE ADMITTED TO THE SEALED CHAMBER.

July 27, 1971 D- H GRANTHAM ETAL 3,595,775

SPUTTERING APPARATUS WITH SEALED CATHODE-SHIELD CHAMBER Filed May l5,1969 Li \Mm f% f7 P 'Il l QQ@ *gygy mmm United States Patent O 3,595,775SPUTTERING APPARATUS WITH SEALED CATHODE-SI-HELD CHAMBER Daniel H.Grantham, Glastonbury, Daniel I. Quinn, Manchester, and Edouard L.Paradis, Wllimantc, Conn.,

assgnors to United Aircraft Corporation, East Hartford, Conn.

Filed May 15, 1969, Ser. No. 824,929 Int. Cl. C23c 15/00 U.S. Cl.204-298 8 Claims ABSTRACT F THE DISCLOSURE vacuum pump in a sealedchamber between the cathode and the ground shield to prevent arcing athigh power densities. Atmospheric pressure may also be admitted to thesealed chamber.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a high rate sputtering apparatus for depositing lms ofmaterial on a substrate. More specifically, this invention relates to asputtering apparatus for conducting, semiconducting and insulatingmaterials in which high power densities may be applied to the materialto increase the rate of sputtering. A sealed chamber is provided betweenthe cathode electrode and its associated ground shield which may beevacuated by a vacuum pump to prevent arcing and breakdown between thecathode and the ground shield. Atmospheric pressure may also be admittedto the sealed chamber. A magnetic eld having a high radial component maybe applied to the target to increase the rate of sputtering.

Description of the prior art U.S. Pat. No. 3,369,991 describes anapparatus for cathode sputtering of solid dielectric materials includinga shielded RF electrode. An important feature of the patent is thespacing provided between the exposed surface of the target electrode(cathode) and its shield. More specically, the patent states that whenthe spacing lies within a certain critical range, an`optimum conditionis achieved whereby the radio-frequency capacitive coupling between theelectrode and its shield is low enough to be disregarded, yet there isno perceptible tendency to sputter metal or other contaminants `from theelectrode. The spacing has a lower limit of about one-quarter inch andan upper limit not greater than the Crooks dark space in the glowdischarge. Thus the radio-frequency sputter'- ing of dielectrics must beaccomplished with gas pressure no greater than about 30 microns. Atypical example uses 5 microns pressure in argon gas.

A further feature disclosed in the prior art patent is the applicationto the gas in the ionization chamber of a steady magnetic field toenhance the ionizing action. The magnetic iield is applied normal to thesurface of the target.

However, with the apparatus as described in the prior art patent, theapplication of high magnetic elds and large voltages in order toincrease the sputtering rate results in an arc breakdown at the cathode..The sputtering rate is limited by the high voltage breakdown of the lowpressure gas between the electrode and the grounded shield in proximityto it.

From Paschens law it is known that gaseous breakdown voltages passthrough minimum values and then begin ICE.

to increase rapidly as the pd product decreases, where p is the gaspressure and d is the electrode to ground shield spacing. The arcbreakdown problem at high power densities cannot be overcome by anarbitrary reduction in the pd product by decreasing the spacing dbetween the electrode and the shield because of the fabricationtolerances and the onset of other breakdown mechanisms such as fieldemission from microscopic irregularities in electrode and shieldsurfaces. Furthermore, with too close coupling between the electrode andthe ground shield, excessive RF power would be lost to ground.

SUMMARY OF THE INVEN'DION It is therefore a primary object of thisinvention to increase the rate of sputtering of conducting,semiconducting and insulating materials in a sputtering apparatus bypreventing arc breakdown between the cathode electrode and its shield athigh power densities.

In accordance with the present invention, a sealed chamber is formedbetween the cathode electrode and its grounded shield through the use ofappropriate seals, and the chamber is evacuated by a vacuum pump to apressure below about 5 101r5 torr. This reduces the pd product in theannular space around the cathode and permits power density inputs toithe target of 850 watts/in.2 and above.

In accordance with another aspect of the present invention, a gas at orabout atmospheric pressure may be admitted to the sealed chamber betweenthe target electrode and the ground shield. This places the operatingconditions in a higher voltage breakdown region above the minimum valueon Paschens curve.

In accordance with a further aspect of this invention, a high properlyvectored magnetic field is applied to the target to increase the plasmadensity and optimize the sputtering rate. An electromagnet in the shapeof a toroid surrounds the target and produces a magnetic eld at thetarget which has both axial and radial components with respect to thetarget, thereby increasing the electron path length between theelectrodes and increasing the collisional probability.

The sealed chamber and the magnetic eld are applicable to all types oftargets, and to embodiments in which either DC or RF electric elds areapplied between the electrodes.

BRIEF DESCRIPTION OF THE DRAWING The figure is a vertical sectionshowing the shielded electrode structure of a dielectric sputteringapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT A general description of theprocess of sputtering a dielectric material and coating the dielectricupon a sample may be found in U.S. Pat. 3,369,991. This patent alsodescribes the problems which arise when it is attempted to shield thetarget electrode from bombardment by the gas ions. This prior art patentteaches that a space is provided between the exposed surface of thetarget electrode and its shield, the spacing lying within a certaincritical range whereby the RF capacitive coupling between the electrodeand its shield is low enough to be disregarded, yet there is noperceptible tendency to sputter metal or other contaminants from theelectrode.

The apparatus described in this prior art patent has been found to bedeficient at high power densities and high magnetic fields when it isattempted to increase the rate of sputtering. The deiiciency involvesthe breakdown and arcing of the gas in the space between the targetelectrode and the shield.

In order to overcome this deficiency, this invention provides a closedchamber between the target electrode and its shield. In the preferredembodiment this chamber is evacuated to a very low pressure. The spacebetween the target electrode and the shield is therefore not open to thehigher gas pressures in the sputtering chamber. By lowering the gaspressure, much higher power densities and magnetic fields may bemaintained and the rate of sputtering increased by a factor of about l0.

In another embodiment of this invention, the sealed chamber between thetarget electrode and its shield has admitted thereto a high pressure gasat about atmospheric pressure or slightly above. This embodiment allowsfor the application of lhigher electric and magnetic fields at thetarget than the prior art, but not as high as when the sealed chamber isevacuated to a very low pressure. The advantage of using the highatmospheric pressure is that the apparatus is simpler and less expensivethan in the evacuated embodiment.

The invention will be described in its preferred embodiment as using adielectric target in a configuration which requires RF electric fields,but it will be apparent to one skilled in the art that other types oftargets such as conducting and semiconducting may be used, and likewisethat either DC or RF electric fields may be used.

Referring to the ligure there is shown a vertical section view of theshielded electrode structure of this invention. A low porositydielectric target is mounted on the cathode electrode 12. The electrode12 is preferably brazed in one piece to prevent bowing of the frontsurface. A soft solder seal 14 is provided at the interface between thetarget 10 and the cathode 12. The cathode 12 contains chambers 16through which a cooling uid may be distributed as 4will be described.

Surrounding the cathode 12 is a ground shield 18 of metallic materialhaving an upwardly extending annular lip portion that fully encloses thecathode 12. An O ring 20 is inserted between the metallic shield 18 andthe target 10 to provide a vacuum seal for chamber 22 between the shield18 and the cathode 12. The shield 18 is maintained at ground potential.

The target 10 is maintained in its position by a sealing ring or flange24 which is fixed in position by bolts 26 extending through shield 18.

Ground shield 18 is cooled by a plurality of cooling coils 28 throughwhich a cooling fluid is distributed from a fluid source (not shown).

Shield 18 is supported by one end of a grounded post 30, the post beingsupported at its other end by an annular metallic ring 32 which iselectrically connected to ground. Connected to metallic ring 32 is aninsulating ring 34 whose purpose is to center and support metal tube 36and tube 38 concentric with tube 36 through which cooling fluid flows tocathode 12. An O ring seal 40 is positioned between metallic ring 32 andinsulating ring 34.

Metallic ring 32 and insulating ring 34 are maintained in rigid positionrelative to each other by bolts 35. Seal 40 is compressed thereby toeffect a vacuum seal for chamber 22. Threaded metal sleeve 42 centerstubes 36 and 38 with respect to post 20, and exerts pressure on O ringseal 46 to provide vacuum sealing of chamber 22 when ferrule 44 with itsseal 48 are threaded onto sleeve 42.

A low pressure gas ionization chamber is provided for the sputteringoperation by a wall 50 in the form of a bell jar made of a suitablematerial. Access to the chamber is gained by penetrating the wall 50 andeffecting a vacuum seal by means of a ange 52 which is sealed by seal53. Flange 52 is threaded at one end and is supported by a ferrule S4sealed at 55.

Inserted in the low pressure ionization chamber is an anode 56, havingcooling passages 57. A radio-frequency voltage is applied between anode56 and cathode 12 from an RF source, not shown, which is connected tothe cathode through lug or terminal 58. The other side of the RF sourceis connected to anode 56 by any suitable means.

Terminal 58 is in electrical contact with cathode 12 through metal tube36. Insulating ring 34 maintains electrical insulation between thecathode voltage and the voltage of the grounded shield 18 and metal post30.

A cooling fluid is flowed through tube 38 to cathode 12 through an inputtube 60. The cooling iiuid flows through chambers 16 within the cathodeand is returned through tube 36 and outlet tube 62 to a reservoir.

Bellows 64 and 66 are provided in the lines of tubes 36 and 38 toprevent stress on target 10 from misalignment in the final assembly andthe expansion and contraction caused by the high power operation andheating of the apparatus.

Threaded ferrules 68 and 70 sealed by gaskets are provided to supportlines 60 and 62 at the inlet and outlet for the cooling liuid.

The sample 76 to be coated may be located between anode 56 and cathode12, and preferably is in contact with anode 56. The coating takes placein the ionization chamber at reduced pressures and in suitable gasatmosphere as is known to those skilled in the D In the preferredembodiment of this invention, a vacuum pump, not shown, is connectedwith chamber 22 through an opening 72 in the wall of post 30. Thepressure in sealed chamber 22 is reduced to a value below about 5X 10-5torr by the vacuum pump. At this pressure there is no breakdown betweenthe cathode 12 and the shield 18 at power densities of approximately 850Watts/in?. By virtue of the sealing arrangement shown, chamber 22 may bemaintained in a highly evacuated condition since it is not in contactwith either atmospheric pressure or with the fairly low pressure withinthe ionizing chamber in which the sputtering is performed.

Alternately, the sputtering apparatus may be operated by admitting highpressure gas into the chamber 22, the gas being at atmospheric pressureor slightly above. In this mode of operation the device operates on thehigh pd portion of the Paschen curve in a high breakdown region. Thepreferred gases are SP6 and some of the Freons. Air may also be used. Asindicated previously, this mode of operation will allow for theapplication of higher electric and magnetic fields at the target thanthe prior art, but it will not allow for application of as high fieldsas the evacuated mode of operation. However, the elimination of thevacuum pump allows simpler and less expensive construction.

Although the invention has been described in terms of a dielectric or aninsulating target, the invention may also be used with conducting metalsand semiconducting targets by adding electrical insulation between themetal or dielectric target and the ground shield. Appropriate sealing isalso necessary. In these embodiments, either DC or RF electric fieldsmay be used depending upon the preferred configuration. With metallictargets, a portion of the shield may be made from insulating materialsuch as A1203 for DC operation.

Regardless of the configuration, operation may be enhanced bysurrounding the cathode and cathode shield with an electromagnet 74 inthe shape of a toroid. The exact position of the electromagnet 74 in theaxial direction and its precise size may be varied somewhat depending onthe mode of operation of the sputtering apparatus. Optimum couplingbetween the magnetic field and the electric field between the cathodeand anode is desired. It is important that the magnetic field have ahigh radial component so that the electron path length between thecathode and anode electrodes may be increased. For some modes ofoperation it is desirable that the radial component of the magneticfield be about the same order of magnitude as the axial component of themagnetic field. The reason for the magnetic field is that at pressuresof between about 4 and l2 microns, electron mean free paths are on thesame order of magnitude as the electrode separation. Therefore, toincrease the probability of collision, the electron path length betweenthe electrodes is increased by the radial field as the electrons areaccelerated toward the anode by the electric iield. The axial componentf the field tends to prevent the escape of those electrons which have aradial velocity component.

Another important aspect of this invention is that the metal cathode 12is bonded to the target material 10 with an appropriate solder 14, andthe cathode 12 is cooled by water or some other iiuid in order to keepthe target temperature uniform and to avoid cracking. Flange 24 aroundthe target is designed to avoid torsional stress in the target, which,coupled with the temperature gradient through the target, has in thepast caused cracking at relatively low power densities.

Sputtering rates have been increased considerably by use of theapparatus described. Power densities using RF fields have been raised toapproximately 850 watts/in.2 and above at high RF frequencies with thepressure in the chamber 22 lowered to 5x10-5 torr and below. Rates ofdeposition for representative materials are: A1203, 6,300 angstroms perminute; for SiOZ (quartz), 10,000 angstroms per minute.

It is to be understood that the invention is not limited to the specificembodiment herein illustrated and described but may be used in otherways without departure from its spirit as dened by the following claims.

We claim:

l. In an apparatus for coating samples with material sputtered from atarget7 and including:

a low pressure gas ionization chamber adapted to contain the target andthe sample to be coated,

means for applying an electric eld between first and second electrodesin said ionization chamber for ionizing said gas and maintaining a glowdischarge, said target being supported by said first electrode and beingmounted substantially between said electrodes, and a grounded shieldsurrounding said first electrode,

the improvement comprising a sealed chamber between said first electrodeand said shield, a portion of said sealed chamber being formed by saidtarget,

and means for maintaining a preselected gas pressure in said sealedchamber to prevent breakdown between said irst electrode and saidshield.

2. Apparatus as in claim ll and including means for applying a magnetic'field to the ionized gas in said ionizing chamber.

3. Apparatus as in claim 2 in which said magnetic field has a iirstcomponent in the direction normal to said target, and a second componentin the direction at right angles to said rst component, said twocomponents being approximately equal.

4. Apparatus as in claim 2 in which said means for applying a magneticfield is a toroidal electromagnet positioned substantially adjacent saidtarget.

5. Apparatus as in claim ll and including means for evacuating saidchamber to a pressure much lower than the pressure in said ionizationchamber.

6. Apparatus as in claim ll and including means for pressuring saidchamber with a gas at about atmospheric pressure.

7. Apparatus as in claim 1 in which said means for applying saidelectric field is a DC source.

8. Apparatus as in claim 1 in which said means for applying saidelectric eld is a RF source.

References Cited UNITED STATES PATENTS 2/1968 Davidse et al. 204-298 3/1970 Humphries 204-298

